How does a garage door lock when fitted with an electric opener?
There are various ways a garage door will lock with an electric operator fitted but it depends on the door and of course the operator.
A Basic Installation
In a basic installation of a remote control electric operator to a standard one piece up and over retractable type garage door it is common to simply remove the internal locking cables or rods, as these are nearly always a slam latch type, so you cannot have the door closing automatically and then being locked internally preventing an automatic opening cycle. This would make no sense at all! The removal of the locking system means the garage door is now held closed firmly by the operator towing arm only at the top, and this is a solid piece of steel bar connected to the top of the door panel so relies on the strength and integrity of the door panel itself.
In all up and over garage doors there will be a certain amount of panel flex so when the door handle or the bottom of the door is pulled hard there will be some movement, greater in older doors and some more than others depending on the chassis and frame construction. This movement gives the impression to a would be intruder, trying the handle maybe, that some more force would enable a break in, and yes this is possible of course, but not without a great amount of noise and damage first, and again some garage doors are simply stronger than others in their chassis construction so would resist a lot longer. This basic installation is fine if you use a good quality electric operator as many have automatic functions that keep checking the door is held down firmly and some also have built in locking latches in the boom. Check the security method offered first is you have any concerns at all.